package com.bestdreamer.url;

import java.io.InputStream;
import java.io.RandomAccessFile;
import java.net.HttpURLConnection;
import java.net.URL;

/**
 * 多线程下载工具类
 * download() 方法实现多线程下载步骤：
 * 1、创建 URL 对象
 * 2、获取指定 URL 对象所指向资源的大小（通过 getContentLength()方法获得），此处用了 URLConnection 类，该类代表 Java 应用程序和 URL 之间的通信链接。
 * 3、在本地磁盘上创建一个与网络资源具有相同大小的空文件
 * 4、计算每个线程应该下载网络资源的哪个部分（从哪个字节开始，到那个字节结束）
 * 5、依次创建、启动多个线程来下载网络资源的指定部分。
 *
 */
public class DownUtil {

    // 定义下载资源的路径
    private final String path;
    // 指定所下载的文件的保存位置
    private final String targetFile;
    // 定义需要使用多少个线程下载资源
    private final int threadNum;
    // 定义下载的线程对象
    private final DownThread[] threads;
    // 定义下载的文件的总大小
    private int fileSize;

    public DownUtil(String path,String targetFile,int threadNum){
        this.path = path;
        this.threadNum = threadNum;
        // 初始化 threads 数组
        threads = new DownThread[threadNum];
        this.targetFile = targetFile;
    }

    public void download() throws Exception{
        URL url = new URL(path);
        HttpURLConnection conn = (HttpURLConnection)url.openConnection();
        conn.setConnectTimeout(5 * 1000);
        conn.setRequestMethod("GET");
        conn.setRequestProperty(
                "Accept",
                "image/gif,image/jpeg,image/pjpeg,image/pjpeg,"
                        + "application/x-shockwave-flash,application/xaml+xml,"
                        + "application/vnd.ms-xpsdocment,application/x-ms-xbap,"
                        + "application/x-ms-application,application/vnd.ms-excel,"
                        + "application/vnd.ms-powerpoint,application/msword,*/*");
        conn.setRequestProperty("Accept-Language","zh-CN");
        conn.setRequestProperty("Charset","UTF-8");
        conn.setRequestProperty("Connection","Keep-Alive");
        // 得到文件大小
        fileSize = conn.getContentLength();
        conn.disconnect();
        int currentPartSize = fileSize / threadNum + 1;
        RandomAccessFile file = new RandomAccessFile(targetFile,"rw");
        // 设置本地文件大小
        file.setLength(fileSize);
        file.close();
        for( int i = 0; i < threadNum; i++){
            // 计算每个线程下载的开始位置
            int startPos = i * currentPartSize;
            // 每个线程使用一个 RandomAccessFile 进行下载
            RandomAccessFile currentPart = new RandomAccessFile(targetFile,"rw");
            // 定位该线程的下载位置
            currentPart.seek(startPos);
            // 创建下载线程
            threads[i] = new DownThread(startPos,currentPartSize,currentPart);
            // 启动下载线程
            threads[i].start();
        }
    }

    // 获取下载的完成百分比
    public double getCompleteRate(){
        // 统计多个线程已经下载的总大小
        int sumSize = 0;
        for(int i = 0; i < threadNum; i++){
            sumSize += threads[i].length;
        }
        // 返回已经完成的百分比
        return sumSize * 1.0 / fileSize;
    }

    private class DownThread extends Thread{
        // 当前线程的下载位置
        private final int startPos;
        // 定义当前线程负责下载的文件大小
        private final int currentPartSize;
        // 当前线程需要下载的文件块
        private final RandomAccessFile currentPart;
        // 定义该线程已下载的字节数
        public int length;
        public DownThread(int startPos, int currentPartSize,RandomAccessFile currentPart){
            this.startPos = startPos;
            this.currentPart = currentPart;
            this.currentPartSize = currentPartSize;
        }
        public void run(){
            try{
                URL url = new URL(path);
                HttpURLConnection conn = (HttpURLConnection)url.openConnection();
                conn.setConnectTimeout(5 * 1000);
                conn.setRequestMethod("GET");
                conn.setRequestProperty(
                        "Accept",
                        "image/gif,image/jpeg,image/pjpeg,image/pjpeg,"
                        + "application/x-shockwave-flash,application/xaml+xml,"
                        + "application/vnd.ms-xpsdocment,application/x-ms-xbap,"
                        + "application/x-ms-application,application/vnd.ms-excel,"
                        + "application/vnd.ms-powerpoint,application/msword,*/*");
                conn.setRequestProperty("Accept-Language","zh-CN");
                conn.setRequestProperty("Charset","UTF-8");
                InputStream inStream = conn.getInputStream();
                // 跳过 startPos 个字节，表明该线程只下载自己负责的那部分文件
                inStream.skip(this.startPos);
                byte[] buffer = new byte[1024];
                int hasRead = 0;
                // 读取网络数据，并写入本地文件
                while(length < currentPartSize && (hasRead = inStream.read(buffer)) != -1){
                    currentPart.write(buffer,0,hasRead);
                    // 累计该线程下载的总大小
                    length += hasRead;
                }
                currentPart.close();
                inStream.close();
            }catch (Exception e){
                e.printStackTrace();
            }
        }
    }

}
